Putting the Metaball panels in the Properties sidebar - object has no attribute meta_ball
Some context properties are only available to the 'PROPERTIES' space, particularly the data types context.mesh, context.speaker context.meta_ball to name a few.... Another would be be context.world. 

Simply use 

metaball = context.active_object.data


and in poll check that

context.active_object and context.active_object.type == 'META'